TRANSMISSION & DISTRIBUTION
Transforming Mexico’s international links1 July 2007
A new USA-Mexico cross-border interconnection has been inaugurated. It is supply reliability driven and employs a GE technology, the variable frequency transformer, that has been available since 2004 but is only now finding its second commercial application.
The new back-to-back link, designed to smoothly connect the asynchronous transmission systems of Texas and Mexico, was inaugurated in May by state and local officials along with representatives of several partners involved in the project. It should strengthen the connection between Texas and Mexico, and provide enhanced reliability for the whole Laredo area. It will also reduce the need to operate power generation in the area by bringing in shared sources from Mexico. The heart of it is a GE Energy $74 million variable frequency transformer, the first installation of its kind in the US and the second in the world, which will allow for the import or export of 100 MWe.
The Laredo power plant has a higher ‘must-run’ reliability level according to an agreement with the Electric Reliability Council of Texas (ERCOT), one of the main power grid associations in the country. Carl English, president of AEP Utilities, said "The big picture is the connection between Mexico and the United States. It gives both of us greater reliability for the future and gives the ability to have more electric commerce. The growth of population and business puts a strain on our electric systems, particularly in this area which is one of fastest growing areas of the country. That's a very serious consideration. The more we help each other out and can back each other up, the better of we will be and well as Nuevo Laredo."
The demand for electricity continues to grow in Laredo and the surrounding area which necessitates new methods of supplying power to the region. The availability of the VFT provided one of the methods designed to meet this growing demand. The project was a collaboration among AEP, MidAmerica Energy, GE Energy, which designed, manufactured and commissioned the VFT, and the Commisión de Electricdad of Mexico. It took four years to get started and two years to get operational.
The new VFT-based connector is part of a larger plan to bring more power to south Texas and northern Mexico. There are plans to address the growth of Laredo in the long term, but it has taken years to get this far with building the planned 345 kV line now approved by ERCOT, whereas the VFT is addressing the problems of increased demand right now. The 345 kV line is not scheduled to be completed until 2010.
The variable frequency transformer is based on a combination of hydro generator and transformer technologies. It is a controllable, bi-directional transmission device that can transfer power between asynchronous networks under all operating conditions.
A common situation is where two grids of the same nominal frequency cannot economically be directly connected with AC lines. The VFT allows controlled power exchange between the grids, while retaining many of the inherent virtues of an AC interconnection.
Functionally, the VFT is similar to a back-to-back HVDC link. The core technology is a rotary transformer with three-phase windings on both rotor and stator (Figure A). A motor and drive system are used to adjust the rotational position of the rotor relative to the stator, thereby controlling the magnitude and direction of the power flowing through the transformer.
The first VFT was installed and commissioned for TransEnergie, Hydro Québec's transmission division, at its Langlois substation in 2004, where it is being used to exchange up to 100 MW of power between the asynchronous power grids of Quebec, Canada) and New York, USA.
The technology is a direct competitor to existing back-to-back solutions using HVDC or VSC configurations, and is claimed to be particularly applicable to regions with weak and geographically dispersed grids, such as those sometimes found in the Middle East, Africa and southern Asia.
There is a now body of operational experience from Langlois as well as simulation and test results for the AEP Laredo back-to-back. Figures 2 and 3 shows results from validation tests conducted during the commissioning of the Langlois substation. Two Beauharnois machines were synchronised to the Cedres substation in the islanded configuration shown in Figure 2 (Cedres – Langlois line opened and Dorian load fed from another bus). The total production of the two machines in the island was set at 45 MW (35 MW and 10 MW). Figure 3 shows the power command (P CMD) and measured power through the VFT compared to the system simulation (P PSS). Results agree very well.
This project is one of several reliability-must-run (RMR) exit-strategy projects for the Laredo power station and is endorsed by the ERCOT board for the purpose of maintaining reliable power supply to the area. In the past, local generation supported the Laredo area, but that changed in 2002 when AEP notified ERCOT of its intention to shut down the Laredo power plant owing to adverse market conditions. ERCOT responded by entering into an RMR agreement with AEP to keep the units operating until power-delivery system improvements could be constructed. AEP then began studying short-term solutions to reduce the unit run times until the new 345 kV transmission line could be brought on-line in 2010.
The short-term plan required that a fifth
138 kV source be linked to the Laredo area. Given the presence of a strong 138 kV CFE network just across the border, and the cross-border
138 kV infrastructure already in place, an asynchronous tie to CFE made sense as an expedient way of providing this fifth source in the short-term .
VFT core technology
At the heart of the VFT is a rotary transformer with three-phase windings on both the rotor and stator sides. The collector system conducts current between the three-phase rotor winding and its stationary bus. In the case of the Laredo VFT system, the rotor side of the VFT is connected to the CFE grid and the stator side to the ERCOT grid, but the arrangement is arbitrary.
Power flow is proportional to the magnitude and direction of the torque applied to the rotor, the torque being applied by a drive motor controlled by a variable-speed drive system. If torque is applied in one direction, then power flows from the stator windings to the rotor windings. If in the opposite direction, then power flows from the rotor windings to the stator windings. If no torque is applied, then no real power flows through the transformer.
A closed-loop power regulator maintains power transfer according to the operator setpoint by adjusting motor torque as a function of power error, and can respond quickly to network disturbances. Regardless of power flow, the rotor automatically orients itself to follow the phase angle imposed by the two asynchronous systems. If the grids are at different frequencies then it rotates continuously.
The motor and drive system are designed to continuously produce torque even when stationary. If the power grid on one side experiences a disturbance that causes a frequency excursion, the VFT rotates at a speed proportional to the difference in frequency between the two power grids. During such a disturbance, if the VFT is transferring power, it will continue without interruption and at full expected power, that is, it can continuously regulate power flow despite drifting frequencies on both grids.
Reactive power flow through the VFT follows conventional ac circuit rules. It is determined by the series impedance of the rotary transformer and the difference in voltage magnitude on the two sides. But (unlike power-electronic alternatives) the VFT produces no harmonics and cannot therefore cause undesirable interactions with other generators or equipment on the grid.
Evaluation of the interconnection compared three different asynchronous links – conventional HVDC back to back, VSC BTB and VFT (Table 1). Dynamic models from individual suppliers were integrated separately into the base study case, providing a basis for comparative performance evaluations.
Because the Laredo area is vulnerable to dynamic voltage collapse, particularly during summer peak-load conditions, the evaluation tests included power-flow and dynamic-stability analysis. In the dynamic studies, detailed modelling of the ERCOT transmission network and connected generation was included. Existing FACTS devices — including the Laredo and Military Highway ±150 MVAR STATCOM and the Eagle Pass 36 MW VSC BTB tie — were modelled. Alternate representations of the CFE system also were considered but did not produce useful extra data.
Special attention was paid to modelling the South Texas area load, in particular simulation of the dynamic behaviour of the concentration of air conditioning load typical of a summer demand peak. To accomplish this, an aggregate load model was derived from estimated load class percentages, typical summer peak composition data, and typical load device modelling data and showed that.two-thirds of the load represented as dynamic induction machines and one-third as a static polynomial-type load model.
The VFT and VSC BTB devices showed an increasingly substantial benefit as area load was increased compared to the conventional HVDC BTB. The VFT at 100 MW was observed to have a small but consistent advantage over the 150 MW VSC BTB. While the latter’s transient injection of reactive power helped to stabilise system voltage, the VFT combined transient flow of real and reactive power was more beneficial even though the overall reactive injection is significantly less than that of the VSC BTB. The benefit of supplying an immediate real power boost into the faulted system is that it helps to keep the Laredo area voltage angle from lagging further behind the ERCOT system.
It was also noted that the VFT could supply a stability benefit with zero-scheduled steady-state power flow at the 2007 peak load (469 MW). This is a benefit of the inherent characteristic of the VFT that makes it look more like a phase-shifting transformer than does a conventional HVDC BTB or a VSC BTB. As a result, a transient real and reactive power boost through the VFT is attainable at zero steady-state flow. The VSC BTB also can remain in service and supply reactive power under these conditions but a conventional HVDC system must be turned off completely.
The ability of the asynchronous device to ride through severe voltage depressions is essential for this application. An interruption of the device transmitting capability during a post-fault voltage-recovery period could result in a system collapse. This was also a requirement for the Langlois VFT. GE Energy demonstrated this capability via extensive simulator testing, and there have been field events that substantiate the expected behaviour from that installation.
Steady state loading
Steady state system requirements are related to the thermal capacity of the 138 kV transmission system connecting Laredo with the rest of ERCOT. The import capability offsets the need to import power from ERCOT, relieving the 138 kV network loading, so the steady-state power-import requirements are a simple function of the link’s capacity.
The conventional HVDC BTB and VSC BTB devices, rated at 150 MW, were therefore better in this regard compared to a single 100 MW channel VFT. However, further power-flow studies showed that a 100 MW injection was still adequate to achieve the required power import relief within the study time period. Consequently the steady-state system requirement was not a key selection factor.
Continued operation of the Laredo power station helps to stabilise system voltage-collapse tendencies but is dependent on favourable market conditions and economic factors that cannot be guaranteed. While both the VFT and the VSC BTB were able to achieve necessary stabilisation throughout the study period without running the Laredo plant , the VFT was selected because it offered better overall performance with regard to the stability requirements. The best-performing conventional HVDC BTB device was unable to achieve system stabilisation beyond 500 MW of total area load, so running the Laredo power plant would be necessary in the latter years of the study period as demand increased.